The present invention relates to a steel wire and spring having superior fatigue properties and to a method of manufacturing such a steel wire and spring.
Spring steel wires containing 0.6-0.8 mass % of C, 0.15-0.35 mass % of Si, and 0.3-0.9 mass % of Mn are known in the art. Such a steel wire is manufactured by being processed through steps of rollingxe2x86x92patenting (heating for xcex3-phase transitionxe2x86x92isothermal transformationxe2x86x92wire drawingxe2x86x92(coiling: when to be worked into springs)xe2x86x92strain relief annealing (at 300xc2x130xc2x0 C.).
However, it is rather difficult to say that such spring steel wires as mentioned above are satisfactory neither in thermal resistance nor in fatigue strength. Meanwhile, it is known in various steel wires including parallel wire that thermal resistance may be improved by increasing the Si content. In this respect, however, the purpose of using steel wires having a good thermal resistance varies with their specific uses, the thermal resistance for the case of parallel wire essentially aims at limiting the change in tensile strength (TS) of the wire small when subjected to galvanization (at 450xc2x0 C. for 30 seconds). On the other hand, in the case of those springs associated with automobile engines for which the steel wire of the present invention is intended, important considerations include keeping the permanent set in the temperature range of about 100-200xc2x0 C. small and at the same time providing desired fatigue properties. Thus, simply applying a chemical composition of such a parallel wire to a spring wire cannot bring forth satisfactory properties sufficient for a spring material. That is to say, while the Si addition in a parallel wire is reportedly said to be effective in improving its fatigue properties, this is mere a story of fatigue under repeated tension, which differs essentially from the fatigue properties required for a spring material. It has been shown that a decrease in surface hardness greatly affects the fatigue properties in a spring steel wire having a high Si content, although its influence on the fatigue properties is small in a parallel wire.
It is also known that a steel wire superior in both thermal resistance and fatigue strength (oil-tempered wire) can be obtained by applying quenching and tempering in the final stage of the steel wire manufacture, such a quenching and tempering process adds to the cost.
Accordingly, it is a primary object of the present invention to provide a steel wire and spring having a high thermal resistance and a high fatigue strength that can be produced without applying a quenching and tempering process, namely, produced through a drawing process and a method of manufacturing such a steel wire and spring.
The present invention provides a steel wire comprising a pearlite structure containing 0.8-1.0 mass % of C and 0.8-1.5 mass % of Si, wherein in the cross section of the steel wire the average hardness in an outer region up to 100 xcexcm from the surface thereof is at least 50 higher than that of a deeper region based on micro-Vickers hardness. This steel wire has a high thermal resistance and fatigue strength, and is particularly suited for spring steel wire. Particularly, it is preferable that the deeper region have an average hardness of 500 or above with the outer region having an average hardness at least 150 higher than that of the deeper region based on micro-Vickers hardness.
Preferably, the steel wire may further contain 0.03-0.1 mass % of Mo. Further, it may contain 0.3-0.9 mass % or less Mn and/or 0.2 mass % or less Cr. For providing a sufficient fatigue strength, this steel wire preferably has a tensile strength above 1,900 N/mm2. In addition, it is preferable the steel wire have a residual surface compression stress of 300 MPa or above.
Further, a method of manufacturing the steel wire according to the present invention is characterized by comprising the steps of: shaving a steel wire of pearlite structure containing 0.8-1.0 mass % of C and 0.8-1.5 mass % of Si; patenting the resultant steel wire, and drawing the patented steel wire; processing the resultant drawn steel wire through a strain relief annealing at 350-450xc2x0 C.; subsequently subjecting the thus processed steel wire to a shot peening process. This method of manufacture can produce the steel wire of the present invention without resorting to a quenching and tempering process, and can produce a steel wire having a high thermal resistance and fatigue strength at low cost.
For working the steel wire into a spring according to the present invention, a coiling process may be interposed between the drawing and strain relief annealing processes mentioned above. It may also be preferred to provide a nitriding process subsequent to the strain relief annealing. Further, it may be preferable to provide a secondary strain relief annealing at around 250xc2x0 C. after the above-described shot peening or following the nitriding and the succeeding shot peening processes.
Hereinafter, the aforementioned features of the present invention will be discussed further in detail.
Chemical Composition
C: The lower limit of the C content was determined based on the fatigue strength, while its upper limit was determined based on the wire drawability.
Si: Si is a chemical element essentially required for improvement of thermal resistance. With its content lower than the previously mentioned lower limit no sufficient thermal resistance will be achieved, while the resultant steel wire becomes susceptible to surface flaws if the Si content is higher than its upper limit.
Mo: With an Mo content lower than its lower limit described above it will have a smaller effect on the improvement in the thermal resistance and fatigue strength of the steel wire, while its content exceeding the upper limit will elongate the time required for patenting, resulting in a lowered productivity.
Mn: Mn is added for improving the quench hardenability of steel wire. Mn content exceeding the upper limit tends to increase segregation and lowers wire drawability.
Cr: The aforementioned upper limit is determined, because a longer patenting time becomes required with a Cr content exceeding that level, thus resulting in a lowered productivity.
Shaving
A purpose of the shaving process is to remove a low hardness layer on the surface of steel wire. The fatigue properties are improved by removing those outer layers having a micro-Vickers hardness at least 50 lower than that of the inner portion of steel wire.
Strain Relief Annealing
The strain relief annealing process is applied at 350-450xc2x0 C. for improving the fatigue properties of resulting springs. By annealing at temperatures in this range, strains of the steel wire caused in the course of its drawing and coiling processes can be effectively removed. Such high temperatures to which the steel wire is exposed during its strain relief annealing does not lower the strength of the resultant steel wire because of its Si content. An annealing temperature below the lower limit has only a little effect on fatigue properties improvement, while the strength and fatigue strength of wire both decrease if the annealing temperature exceeds its upper limit. A preferable annealing time may be about 20 minutes in view of effects and productivity.
Shot Peening
To secure a high fatigue strength, a spring wire requires a high surface hardness and a large compression stress. Since the strain relief annealing substantially removes strains from the steel wire, it becomes easier for a shot peening process to impart a stress to the wire in process, and thus the resulting steel wires and springs can have excellent fatigue strength.
Nitriding
When subjected to nitriding for imparting a residual stress, the prior art piano wires will have a decreased strength in its matrix structure and therefore such piano wires cannot have a sufficient residual stress even when treated through nitriding and shot peening. Since the steel wire with an increased Si content according to the present invention has an improved heat resistance and undergoes only a small reduction in matrix strength, the compression stress imparted can effectively contribute to the improvement of fatigue strength.